I updated to 4.99.37 today -- remotely rebooted with a new kernel
built based on sources from around 10 am CET, updated userland (from
4.99.36) and the network connection hung (very shortly after
installing the new userland).

When I reached the console, there was a login prompt and I could type
my user name, but when I pressed enter, the password prompt didn't
come. There were no kernel messages on the console.

The USB keyboard worked (ENTER produced a newline), but I couldn't
login. CTRL-ALT-ESC didn't enter ddb (is the amd64 combination
different or doesn't it work with USB keyboards?).

Pressing the power button shut down the system without problems.

I've now downgraded to 4.99.36/20071117, which had been stable (even
survived a bulk build without problems).

Any ideas what the problem might have been?

Cheers,
 Thomas